Rain Forecast as Hourly Graph

A couple of common questions we all have are, “When is it going to rain?” and “How much rain will we get?”

There is a tool from the National Weather Service that can give insight into both of those questions. The product is the National Weather Service’s Hourly Weather Forecast Graph. The Hourly Weather Forecast Graph shows forecasts of the chance of rain, amounts over time, and when to expect rain.

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The Hourly Weather Forecast Graph shows hour-by-hour weather forecast conditions for 48 hours. It can be set to show the forecast ahead for up to seven days.

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It breaks weather variables into stacked multiple graphs. Weather variables are grouped by temperature, wind, humidity and precipitation potential, and in other charts as needed, precipitation type.

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Included in the Rain Graph are forecast rainfall amount boxes. They show the amount of rain possible in six hour periods if rain is forecasted. By adding up the amounts shown you have an idea of how much rainfall is possible from a rainfall event. Snow amounts are listed as part of a Snow Graph, if snow is forecasted.

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Rainfall amounts are important for knowing whether you have a good chance to incorporate a fertilizer or herbicide application. Or if you can delay an irrigation, because a weather system will provide enough water to meet crop or pasture water needs.

Here is an example of a recent forecast for Chickasha, Oklahoma and actual rainfall amounts from the Oklahoma Mesonet. The forecast indicated a possible total of 1.36 inches of rain. There was a 28% chance the rain would start at 6:00 PM on Monday, February 22, 2016. At midnight, rain chances climbed to 61%. Then at 6:00 AM Tuesday morning, that probability of precipitation rose again to 73%. That probability held steady until 6:00 PM Tuesday evening, when it dropped to 44%. At midnight, that dropped down to 15%.

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Checking on when and how much rain the Chickasha Mesonet site recorded, the rain started at 1:50 AM, Tuesday, February 23rd.

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The rain ended Tuesday afternoon at 3:00 PM. The total amount collected was 0.78 inches.

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Was the forecast perfect? Nope, but the bulk of the rain came when the forecast probability of precipitation was highest. The rain amount was a decent amount, just under eight-tenths. Chickasha received 58% of a possible forecasted rainfall total.  The time of rainfall was shorter than the total possible, but fit well in the time of highest probability, 61% and 73%.

That rainfall information was important. The forecast let us know that the weather system coming through had a good amount of moisture. Enough moisture to incorporate fertilizer or herbicide, get a seeded crop started, or let us likely skip or delay an irrigation.

Just knowing the rainfall probability, doesn’t tell you how much moisture a system is carrying. Forecasted amounts do.

The National Weather Service Hourly Weather Graph forecast is a valuable tool for planning and taking advantage of what weather systems bring our way. These forecast graphs are based on the National Weather Service’s Point Forecasts that can be interpolated down to 1.6 mile by 1.6 mile grids for all USA states.

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